Guttural pouches
We delve into the mysterious air-filled spaces called the guttural pouches, with Gil Riley MRCVS
All about these mysterious air-filled spaces
YOU probably won’t hear about your horse’s guttural pouches until they cause a problem. Unique to a small number of animal species, these pockets of air are out-pouches of the eustachian tube – the duct that connects the middle ear to the pharynx (throat).
One pouch is located either side of the horse’s head, each with a volume of around 300 to 500ml, and in the healthy horse they contain nothing but air. They are lined with a thin membrane, under which lie some critical structures – major arteries, namely the internal and external carotids; four major cranial nerves; the stylohyoid bone, which divides each pouch into two compartments, and the retropharyngeal lymph nodes.
The role of the guttural pouches has been a source of debate for centuries. Suggestions ranged from them acting as a resonating chamber for the whinny, or improving the horse’s hearing, to being buoyancy aids or simply space fillers. It is now understood that they play a vital role in cooling the arteries that carry blood to the brain.
Strenuous exercise causes the body temperature to rise, but any increase in temperature of the blood flowing to the brain would risk dysfunction of this vital organ. The guttural pouches act as a cooling device to ensure that the blood entering the brain is within a tolerable temperature range – an important adaptation that has ensured survival of the species.
BACTERIAL ATTACK
DISEASES of the guttural pouch, while rare, can involve adjacent structures and precipitate some serious, even fatal conditions. These may involve severe haemorrhage, chronic infection with the bacteria that causes strangles, and problems with swallowing, breathing, facial expression and head posture, resulting from nerve damage.
The most common disease is guttural pouch empyema, caused by a bacterial infection – usually Strep equi var equi, the microbe responsible for strangles. The bacteria gain access to the pouch, typically via the retropharyngeal lymph nodes where the infection is captured and forms an abscess.
“The guttural pouches act as a cooling device”
This then ruptures and spills pus into the adjacent pouch.
The pouch fills with thick pus, which can drain through the eustachian tube into the pharynx – and from there to the nasal passages and out of the nostrils. An infected horse may show fever, depression, nasal discharge and painful swelling in the area where the throatlash would run.
The prognosis is good with medical treatment, which involves daily lavage (flushing) of the infected pouches and antibiotic therapy. Systemically ill horses may also require supportive care, including non-steroidal antiinflammatory drugs to control fever and malaise.
Pus that remains in the guttural pouches for a long period as a result of empyema can become inspissated, or dry out, and form chondroids – firm balls that can be difficult to remove (see box, right).
A horse with chondroids may be normal in every respect, back in full work with no outward signs of ill health, but may remain a carrier for the strangles infection.
Such a horse will have a good immunity himself, but will be able to spread the infection to those that lack immunity. Vets might suspect that one or more cases of chondroids exist at a yard if there are repeated strangles outbreaks.
Empyema and chondroids are usually diagnosed with endoscopy, by inserting a thin, flexible camera into the pouches via the nasal cavity. Radiographs can also reveal the presence of chondroids.
Occasionally, inflammation and infection within the pouches can impair the cranial nerves alongside them, resulting in difficulty swallowing, noisy breathing, or impaired function of the muscles involved in facial expression.
Guttural pouch mycosis, a rare but very serious disease, is caused by a fungus that infects the lining of the pouch – usually on the roof, where the presence of blood vessels creates an attractive environment for fungal growth. Once established, the infection can damage the arteries and nerves underneath the lining.
Unlike in empyema, pus does not build up in the pouches and many of these horses appear quite normal until, suddenly, either haemorrhage or damage to the cranial nerves becomes apparent. Since haemorrhage usually involves the internal carotid artery, large amounts of blood can suddenly pour from the nostrils – which will be rapidly fatal.
A surgical technique in which the affected arteries are occluded (stopped) to stem blood loss has a high success rate, provided the procedure takes place before the horse has lost an excessive quantity of blood.
Nerve damage caused by guttural pouch mycosis varies in severity. If it affects less important nerves, the horse might recover. If pronounced, however, and especially if affecting the nerves involved with swallowing or respiration, chances of recovery are slim. Most horses require intensive nursing and assisted feeding for months before signs resolve. Consequently, in many cases, such horses are put to sleep.
TRAPPED AIR
TEMPOROHYOID osteoarthropathy is caused by infection in the middle ear, which invades the nearby temporohyoid joint. The joint and the stylohyoid bone can become brittle and prone to fracture. As the fracture heals, it forms a callus, which puts pressure on the cranial nerves that run through the guttural pouch.
The nerves most typically affected are the facial nerve, involved in muscle movements of the face, and the vestibulotrochlear nerve, responsible for balance. The horse will develop a droopy ear, eyelid and muzzle on the affected side, and may have a head tilt and signs of incoordination. If mild, the condition can be treated with antibiotics, but more chronic cases may require surgery. Sadly, severe signs are unlikely to resolve fully.
Guttural pouch tympany usually affects foals soon after birth. One or both pouches fill with air, which becomes trapped. Although no definite cause has been determined, it is thought that abnormalities of the lining of the eustachian tube, or of the flap where the tube opens into the throat, may be responsible.
The air distension creates swelling in the throatlash area, so the foal resembles a chipmunk whose cheeks are full of nuts. Affected foals are otherwise in good health, although some might have trouble swallowing.
Treatment is surgical and involves the creation of a new opening from the pouch so the air can escape from the affected side into the normal side, or, when both pouches are involved, into the pharynx.